Hitherto, as photoconductive materials which are used for forming photoconductive layers of electrophotographic photosensitive materials, inorganic materials such as amorphous selenium, selenium alloys, metal compound semiconductors (e.g., oxides, sulfides, selenides, etc., of cadmium, zinc, etc.), organic polymers such as polyvinylcarbazole, etc., and organic compounds such as dyes, pigments, etc., have been known in the art, but a technique of using photoconductive amorphous silicon for the formation of photoconductive layers of electrophotographic photosensitive materials has recently been proposed.
Known methods of forming a photoconductive layer composed of amorphous silicon of an electrophotographic photosensitive material include a method of forming the photoconductive layer by dispersing powdery amorphous silicon in a binder and a method of decomposing a silicon-containing gas such as silane or a silane derivative by glow discharging and depositing the silicon thus formed on a conductive support. The electrophotographic photosensitive materials thus formed have as a fundamental structure a photoconductive layer composed of amorphous silicon containing silicon atom and a hydrogen atom and/or a halogen atom on a conductive support.
The amorphous silicon shows very fast dark decay, whereby the image density becomes lower, and, in extreme cases, it is sometimes difficult to obtain images. For improving these disadvantages of amorphous silicon, it is proposed to form on the photoconductive layer composed of amorphous silicon a surface barrier layer mainly composed of carbon and silicon as described in Japanese Patent Application (OPI) No. 115556/82 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), a surface barrier layer mainly composed of nitrogen and silicon as described in Japanese Patent Application (OPI) No. 58160/82 (corresponding to U.S. Pat. No. 4,394,426), or a surface barrier layer composed mainly of oxygen and silicon as described in Japanese Patent Application (OPI) No. 63546/82 (corresponding to U.S. Pat. No. 4,409,308).
However, the electrophotographic photosensitive material having such a surface barrier layer has the disadvantage that the resolving power thereof is greatly reduced by the repeated application of corona discharging, in particular negative corona discharging under high temperature and high humidity conditions such as typically exist in summer weather. This disadvantage is particularly severe when the value of carbon/carbon+silicon (by atomic ratio) in the surface barrier layer mainly composed of carbon and silicon is less than 0.75.